Rainbowdash is missing. Plain and simple. Her memories are gone, stolen by a powerful force that the elements of Harmony were lucky to escape from with their lives. Rainbowdash doesn't remember who she is, or who the ponies are who keep talking to her in her dreams, or the mysterious memory of one pony in particular. The pony whose memory helps to calm her down and to remember that there are things worth fighting for.

“Ah won’t leave ya. Never.” she exclaimed sharply. “Ah’ll always come home. I promise.”
“Okay.” Big Mac reached out to her one last time. “Ah love you.”
“Ah love ya too.”

A promise uttered one evening. The fear and terror of a single fight. The sorrow and pain afterwards.

Applejack has always come home. What does it mean when she doesn't?